Low frequency electric induction furnaces



July 25, 1961 N. COOKE LOW FREQUENCY ELECTRIC INDUCTION FURNACES Filed Oct; 25, 1958 5 Sheets-Sheet l Q om INvEN-roR NoQmaN COOKE July 25,1961 NCOQKE. 2,993,943

LOW FREQUENCY ELECTRIC INDUCTION FURNACES Filed oct. 25, 195s s sheets-sheet 2 INVENTOR NORMAN Coene Many, 99M@ 5 Jw TORNEY July 25, 1961 N. COOKE 2,993,943

LOW FREQUENCY ELECTRIC INDUCTION FURNACES Filed 0G13. 23, 1958 3 Sheets-Sheet 3 ,l l/ 2/ f? /l l(/ l /f ,l f f /7 2, 23 /5 23 24 INvEN-rok Nofzmm Coene BY uw@ IQ we@ Q4 d@ ATTORNEY United States Patent Y 2,993,943 LOW FREQUENCY ELECTRIC INDUCTION FURNACES Norman Cooke, 47 Roseberry St., Penshurst, New South Wales, Australia Filed Oct. 23, 1958, Ser. No. 769,118 Claims priority, application Australia Oct. 28, 1957 6 Claims. (Cl. 'I3- 29) This invention relates to low frequency electric induction furnaces of the cored type and particularly, but not exclusively, to such furnaces which are of the so-called submerged-resistor type.

An object of the invention is to provide an improved form of furnace having high efficiency.

Accordin-g to the invention a low frequency electric induction furnace of the cored type includes a heat shield comprising one or more evacuable or evacuated chambers located between a primary Winding of the heating element and parts of the furnace body adjacent to the primary winding and ydesigned to reduce transfer of heat from those parts of the furnace body to the primary winding and to the space immediately surrounding the primary winding.

One particularly advantageous application of the invention is to furnaces of the kind in which the primary winding is located within a tunnel extending through the furnace body and in which the heat shield substantially encircles the primary winding and is located adjacent to the interior wall of the tunnel.

In known furnaces of the type referred to the parts of the furnace body adjacent to the primary winding tend to become very hot While the furnace is in use and it is generally necessary to provide cooling for these pants in order to protect the primary winding from excessive temperatures. The cooling commonly in use at present is effected by a stream of cooling air which is caused to llow through a space separating the primary winding from the adjacent parts of the furnace body. Relatively large quantities of heat are, however, thus removed from the furnace by this cooling air and the efnciency of operation of the furnace is thus correspondingly reduced.

With the present invention the evacuable or evacuated chambers provide a heat-insulating barrier which tends to reduce the heat transfer from the furnace body to the primary winding, and/or lto cooling air which may be circulated between this winding and the heat shield, to be a relatively small value.

According to a further feature of the invention the evacuable or evacuated chamber or each of such chambers is in the form of a double-Walled metallic chamber with one voreach of its Walls internally polished.

`:Preferably moreover the evacuable or evacuated chamber or each of such chambers is made of metal of low heat conductivity, for example of a heat-resisting steel, which might be a steel alloy including approximately 25% of chromium and 20% of nickel.

In any event according to a still further feature of the invention there preferably extends across or around the interior of the chamber between and mainly or wholly out of contact with the walls of the chamber and so as to lie at each point in its surface approximately normal to the direction of transfer of heat through the chamber, a heatreflecting Wall, conveniently in the form of a polished sheet of metal, preferably aluminium or copper.

The chamber or each of the chambers constituting the heat shield in an electric induction furnace according to the invention preferably comprises a double-walled partcylindrical metal tube with the Walls of the tube spaced apart to form the evacuable or evacuated chamber and such Walls being circumferentially incomplete so that the tube does not form a closed electrical circuit.

2,993,943 Patented July 25, 1961 Moreover where a heat-reflecting wall in the form of a polished metal -sheet is provided as indicated above, this wall may be disposed nearer to the cooler than to the hotter wall of lthe evacuable or evacuated chamber and may be separated from the cooler wall and supported from it by means of strips or blocks of heat-insulating material.

The chamber or chambers constituting the heat shield in a furnace according to the invention may either be permanently evacuated and sealed or may have associated with them means for evacuating them continuously or at interv-als, say either at intervals during operation or immediately before each operating period of the furnace.

In any event the degree of evacuation or possible evacuation should be as high as is practicable and is preferably such as to provide within the chamber or each chamber a pressure less than one inch of water when the furnace is in use, that is to say when any air remaining within the chamber or chambers is at working temperature.

The invention may be carried into practice in various ways but one construction `according to the invention is illustrated somewhat diagrammatically by way of example in the accompanying drawings, in which:

FIGURE 1 is a longitudinal section of the lower part of a low frequency electric induction furnace embodying the invention, the section being taken on the line I-I of FIGURE 2,

FIGURE 2 is a transverse section on the line II-II of FIGURE l,

FIGURE 3 is an enlarged perspective view partly cut away of the heat-insulating shield used in the furnace, and

FIGURE 4 is a much enlarged partial cross-section of part of the heat-insulating shield on the line IV-IV of FIGURE l.

In the construction shown in the drawings the furnace is a cored low frequency electric induction furnace of the lso-called submerged resistor type intended for the melting, or melting and alloying, of non-ferrous metals and alloys such as copper, copper alloys or Zinc.

The furnace comprises a furnace body generally indicated at 1 `and including an upper part 2 constructed as a Crucible 3 of conventional form below which a tunnel 4 is formed through the centre part 5 of the body, round which tunnel passes a U-shaped channel or loop 6 in a manner also known per se in the kind of furnace in question, the upper ends of the arms of the loop communicating with the interior of the Icrucible 3. The centre part 5 of the body and the crucible are surrounded by a heatinsulating layer 7 enclosed within an outer casing 8.

Extending through the tunnel 4 is the centre limb 9 of a magnetic core having an outer part 10 of generally 1 rectangular form extending around the furnace body as shown. On the centrelimb 9 is wound the primary winding 11 of the heating element, the secondary winding of which is constituted by the metal in the crucible 3 and loop 6. The Winding 10 is conveniently wound upon a sheath 12 of asbestos cement or the like surrounding the limb 9.

As will be seen, the primary winding 11 is separated by a substantial annular space 13 from the surface of the tunnel 4 and disposed in the outer circumferential part of this space 13 in accordance with the invention, is a circurnferentially incomplete annular heat shield.

The heat shield, as indicated generally at 14, is in the form of a double-walled tubular shell of incomplete annular form so that, considered circumferentially, the ends of the shell are separated by a longitudinally extending air gap 15. As shown most clearly in FIGURES 3 and 4 the shell comprises spaced inner and outer circumferential walls 16 and 17, the space between which is closed at the gap 15 by upturned parts 18 by which the walls are united in the manner indicated in an airtight 3 manner as by welding or the like and is closed at the longitudinal ends of the shield by rings 19 to which the walls are similarly united.

The walls 16 and 17 are conveniently formed of a heat-resisting steel such as one includingV approximatelyA 25% of chromium and 20% of nickel and are internally polished, while the rings 19 may be formed of a similar steel and internally polished. i

Secured within the incompletely annular chamber thus formed by the walls 16, 1'7 and the rings 19 is a heat reflector in the form of a sheet of aluminium or copper 20 which is polished on both faces and which, as will be seen, lies closer to the inner wall 116 (that is to say the cooler wall) than to the outer wall 17 and is spaced fnom the inner wall by a series of bars or blocks 21. The reflecting sheet 20 is held in tension by screws 22 screwed into blocks 23 secured to the upturned parts 18.

The chamber 24 formed by the heat-insulating shield is evacuated to an extent such that the air pressure therein when at working temperature during operation of the furnace will correspond to not more than about one inch of water.

It will thus be seen that the heat-insulating shield 14 provides a substantially evacuated chamber interposed between the primary winding `11 and the surface of the tunnel 4, the inner walls of this evacuated chamber being polished while moreover it contains in addition the polished reflector 20 still further to reduce the transmission of heat from the body of the furnace to the primary winding 11.

As Will be seen, in the construction shown the part of the body of the furnace forming the crucible 3 and the part of the outer casing 8 surrounding it are formed separately from and may be detachable from, the centre part 5 of the body of the furnace which constitutes the actual heating element.

Further, the end plates 8 of the casing which enclose the body 5 of the furnace may be made of asbestos cement or other non-electrical conducting material in order that they shall not form an electrical loop liable to become heated by induction from the primary winding.

Whereas in the above description it has been assumed that the interior of the incomplete annular chamber constituting the heat shield 14 is permanently evacuated in a modication means may be provided for evacuating this chamber periodically as and when required, as by providing an appropriate connection at one end by which it can be coupled to evacuating apparatus.

The operation of the furnace is similar to that of known cored low frequency induction furnaces of the submerged resistor type except that, as will be apparent, the transfer of heat from the molten metal to the primary winding and core will be substantially reduced by the heat shield as compared with furnaces not embodying the invention.

Thus the primary winding will be connected to a source of electrical supply of appropriate frequency, say 50 cycles per second, the metal in the loop 6 and the crucible 3 thus constituting the secondary winding of a low frequency transformer which has as its core the magnetic core 9, 10, the metal in the loop 6 thus being heated and melted and, due to the electrodynamic forces which occur in it agitated vigorously (when molten) and caused to circulate to and from the crucible and thus heat, melt and mix a charge of metal loaded into it.

What I claim as my invention and desire to secure by Letters Patent is:

l. A low Ifrequency induction furnace of the cored type, including a heat shield comprising an evacuated chamber located between the primary winding of ythe heating element and parts of the furnace body adjacent the primary winding to reduce transfer of heat from the furnace body to the primary winding, said heat shield substantially encircling the primary winding and being positioned adjacent the interior wall of said body part, said heat shield comprising a double-Walled cylndrical metal tube with the walls of the tube spaced apart to form the evacuated chamber, said walls being circumferentially incomplete, whereby the tube does not form a closed electrical circuit.

2. A low frequency elect-ric induction `furnace as claimed in claim l in which at least one of the walls of the double-walled metal tube is polished on its inner surface.

3. A low `frequency electric induction furnace as claimed in claim 1 including a heat reflecting metal wall extending around the interior of the chamber between and at least mainly out of contact with the walls of said chamber and having at least one of its surfaces polished.

4. A low frequency electric induction furnace as claimed in claim 3 in'which the said heat reflecting metal wall is polished on both surfaces.

5. A low frequency electric induction furnace as claimed in claim l in which the heat shield comprises a double walled metallic chamber with the internal surface of at least one of the rwalls polished.

6. A low frequency electric induction furnace as claimed in claim 5 including a heat reflecting metal wall lying between the two walls of the chamber and at least mainly out of contact with such walls and having at least one of its surfaces polished.

References Cited in the file of this patent UNITED STATES PATENTS Re. 21,618 OLeary Nov. 5, 1940 Re. 22,602 Tama Feb. 13, 1945 Re. 22,948 Tama et al. Dec. 9, 1947 1,069,923 Crafts Aug. l2, 1913 1,453,097 Foley Apr. 24, 1923 2,453,946 Sulfrian Nov. 16, 1948 2,633,414 Boivinet Mar. 3l, 1953 2,643,022 Cornell lune 23, 1953 2,767,962 Blackburn Oct. 23, 1956 

